Electrostatic printing



Odi. 21, 1958 H, G, GElG 2,857,272

ELECTROSTATIC PRINTING Filed Sepl'..l 28, 1954 INVENTOR. ,Ll/wom dfi/6 United States Patent O 2,857,272 LECTROSTATIC PRINTING Harold Grey Greig, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application September 28, 1954, Serial No. 458,896

11 Claims. (Cl. 96-1) This invention relates to electrostatic printing, and particularly to a method for removing the photoconductor from a material comprising a photoconductor dispersed in an electrically-insulating, film-forming Vehicle.

An electrostatic printing process is that type of process for producing a visible record, reproduction or copy which includes as an intermediate step, converting a light image or electric signal into an electrostatic charge pattern on an electrically-insulating base. The process may include the conversion of the charge pattern into a visible image which may be a substantially faithful reproduction of an original except that it may be different in size. An electrostatic printing process using photoconductive layers is described in U. S. Patent 2,297,691, issued October 6, 1942, to C. F. Carlson.

A typical electrostatic printing process may include coating a backing with a photoconductive insulating material comprising a photoconductor, such as White zinc oxide, dispersed in an electrically-insulating, film-forming vehicle, or binder such as a silicone resin, and then providing a blanket electrostatic charge on the surface of the photoconductive material. A light image is focused on the charged surface, discharging the portions irradiated by light, leaving the remainder of the surface in a charged condition, thereby forming an electrostatic image thereon. The electrostatic image is rendered visible by applying to the electrostatic image a developer substance, such as a pigmented thermoplastic resin powder which is -held electrostatically to the charged areas of the sheet. The visible image thus formed is then fixed directly to the photoconductive coating, for example, by fusing the image to the coating.

In the above described process, it is sometimes desirable to use a photoconductive material comprising va colored photoconductor, such as a pink zinc oxide, dispersed in an electrically-insulating, film-forming resin in order to obtain a desired spectral response or a desired speed of response. Similarly, a dye-sensitized photoconductor such as White zinc oxide sensitized with Rose Bengal dye and dispersed in a film-forming resin may have been used. The color of the photoconductor or the dye may, however, be undesirable after the developed image is fixed to the photoconductive coating. For these and other reasons, it is desirable to provide a method for removing the photoconductor from a material compris- `ing a photoconductor dispersed in an electrically-insulating, film-forming vehicle or binder Without disturbing a developed image which is fixed to the surface thereof.

An object of the invention is to provide an improved method for removing the photoconductor from a material comprising a photoconductor dispersed in an electrically-insulating, film-forming vehicle or binder without vdisturbing a developed image which is xed to the surface thereof.

Another object is to provide an improved electrostatic printing process.

A further object is to provide an improved method for fice decolorizing a material comprising a colored photoconductor dispersed in an electrically-insulating, film-forming vehicle or binder. 4

In general, a principal feature of the invention provides an electrostatic printing process including the step of removing the photoconductor from a layer of photoconductive material comprising a photoconductor dispersed in an electrically-insulating, film-forming vehicle or binder and having a developed image fixed to the surface thereof. The method comprises immersing the photoconductive material in a reagent which is a solvent for the photoconductor until the photoconductor is dissolved, the electrically-insulating, film-forming vehicle being substantially unaffected by the reagent.

The invention is more completely described in the following detailed description when read in connection with the accompanying drawing in which the sole figure is a flow sheet illustrating a method for carrying out the invention.

Example 1.-Referring to the sole figure, an electrophotographic recording medium is prepared by coating a sheet of paper with a photoconductive material comprising 2.5 parts by weight of pink zinc oxide having an extremely small particle size (of the order of 0.01 to 0.35 u mean diameter) and dispersed in a solution containing 1.0 part by weight of a silicone resin, such as Dow-Corning 803 marketed by Dow-Corning Corporation, Midland, Michigan. This paint-like mix is coated and dried on a paper sheet to form a layer which is preferably about 0.0005 inch in thickness when dry.

The dried photoconductive coating is provided With a blanket electrostatic charge, for example by exposing the coating to a corona discharge or other electron or ionproducing means. A light image containing the information to be recorded is projected upon the charged surface, discharging the illuminated areas, and leaving the remaining areas of the surface in a charged condition, thus forming an electrostatic image thereon.

The electrostatic image is developed in darkness by applying to the electrostatic image a finely-divided developer substance which is electrostatically-attracted to either the charged or discharged areas depending on the charge polarity of the electrostatic image and the developer powder. It is preferred to apply to the electrostatic image a developer brush comprising a magnet having secured thereto a mixture of magnetic carrier particles, such as iron particles free from grease or other impurities soluble in alcohol, and finely-divided pigmented thermoplastic particles, such as piccolastic resin 4358 (an elastic thermoplastic resin composed of polymers of styrene, substituted styrene and its homologs marketed by the Pennsylvania Industrial Co., Clairton,

Pennsylvania) pigmented with carbon black and iosol black dye.

The developed image is fixed to the photoconductive coating. In the preferred embodiment, the thermoplastic developer particles are fused to the photoconductive coating by applying heat, for example from an infra-red lamp or other heat producing means. lf the photoconductive coating is thermoplastic, the developed image may be fixed in a similar manner.

The electrophotographic paper with the fixed image thereon is immersed in a 20% aqueous hydrochloric acid solution until the zinc oxide is dissolved. This ordinarily takes about two minutes for a coating about 0.0005 inch thick. The sheet is removed from the solution, rinsed in Water and then dried.

The photoconductive coating is ordinarily a light pink to a deep brown in color. After treatment, the remaining coating is White to cream in color; the color being that of the remaining resin and paper backing.

Other acid reagents may be used in place of aqueous hydrochloric acid, for example acetic acid and sulphuric acid. The strength of the reagent is not critical and may be varied over Wide limits. Since zinc oxide isl amphoteric, aqueous solutions of alkali reagents, such as sodium hydroxide, potassium hydroxide, and sodium carbonate may be used to dissolve the zinc oxide. Thus any reagent may be used which dissolves the zinc oxide and which does not substantially affect the silicone resin.

Other electrically-insulating, film-forming vehicles may be substituted for the silicone resin and other photoconductors may be substituted for the zinc oxide. One may remove the photoconductor from a material comprising a photoconductor dispersed in an electricallyinsulating, film-forming vehicle by immersing said material in a reagent which is a solvent for said photoconductor and which does not substantially affect the film-forming vehicle or the developed image fixed thereon. ln the preferred electrostatic printing process, the visible image comprises a pigmented thermoplastic resin which is usually substantially unaffected by the same reagents as the electrically-insulating, film-forming vehicle. v

The resins used as film-forming vehicles are hydrophobic and extremely resistant to wetting and moisture. Since the zinc oxide particles are dispersed in the resin and since the resin is resistant to wetting, one would not expect the acid to dissolve the oxide throughout the coating. According to the invention however, the zinc oxide is completely dissolved leaving only a clear resin layer on the paper sheet. One explanation may be that the thixotropic property of the zinc oxide causes the solid particles in the dispersion to set up in a rigid skeleton structure with the adjacent particles in contact throughout the resin matrix. It has Vbeen observed that, ordinarily, zinc oxide particles protrude from the photoconductive coating. However, since the phenomenon is not completely understood, it is not intended that the invention be limited by any particular theory.

Example 2.-An image-carrying sheet of aluminum foil having a photoconductive coating comprising 2.5 parts by weight of white zinc oxide dispersed in 1 part by weight of a silicone resin and 4containing a small amount of a sensitizing dye such as Rose Bengal is immersed in a aqueous hydrochloric acid solution until the zinc oxide is dissolved. The sheet is removed from the solution, rinsed in water and dried. Before processing, the coating is opaque pink. After processing, the coating is colorless and transparent and the image on the surface of the coating is substantially the same as before the treatment.

Example 3.-An image-carrying sheet of paper having a photoconductive coating comprising 2.0 parts by weight of zinc sulphide dispersed in 1.0 part by weight of a silicone resin is immersed in a aqueous sulphuric acid solution until the zinc sulphide is dissolved. The sheet is removed from the solution, rinsed in Water and dried. Before processing, the coating appears non-porous when observed under a microscope. After processing, microscopic examination indicates that the coating is porous.

Example 4.-An image-carrying sheet of paper having a photoconductive coating comprising 2.5 parts by weight of a photoconductive zinc-cadmium sulphide phosphor dispersed in 1.0 part by weight of a silicone resin is immersed in a 15 aqueous hydrochloric acid solution until the phosphor is dissolved. The sheet is removed from the solution, rinsed in water and dried. Before processing, the coating is yellow-orange in color. After processing, the coating is colorless and the image is unaffected.

There have been described improved methods for removing the photoconductor from a material comprising a photoconductor dispersed in an electrically-insulating, film-forming vehicle, without harm to a developed image on the surface of the material. The methods may be used in improved electrostatic printing processes.

CTL

What is claimed is:

1. In an electrostatic printing process which comprises producing an electrostatic image on a photoconductive material comprising a photoconductor dispersed in an organic, film-forming binder, developing said electrostatic image with a finely-divided developer substance and fixing said finely divided developer substance substantially in situ, the step subsequent to said fixing comprising subjecting said photoconductive material to a reagent which is a solvent for said photoconductor and not for said binder until substantially all of said photoconductor is dissolved, said organic, film-forming binder and said fixed developer substance being substantially unaffected by said reagent.

2. In an electrostatic printing process which comprises producing an electrostatic image on a photoconductive material comprising zinc oxide dispersed in an organic, film-forming binder, developing said electrostatic image with a finely-divided developer substance and fixing said finely divided developer substantially in situ, the step subsequent to said fixing comprising dissolving substantially all of said zinc oxide from said photoconductive material with a reagent which is a solvent for said zinc oxide but not for said organic film-forming binder, said organic filmforming binder and said fixed developer substance being substantially unaffected by said reagent.

3. An electrostatic printing process which comprises producing an electrostatic image on a photoconductive material including a photoconductor dispersed in an organic, film-forming binder, developing said electrostatic image with a finely-divided developer substance, fixing said finely divided developer substance substantially in situ, and then subjecting said photoconductive material to a reagent which is a solvent for said photoconductor and not for said binder until substantially all of said photoconductor is dissolved, said organic, film-forming binder and said fixed developer substance being substantially unaffected by said reagent.

4. An electrostatic printing process comprising coating an electrically-conducting surface with a photoconductive material which comprises a photoconductor dispersed in an organic, film-forming binder, producing a blanket electrostatic charge on the surface of said photoconductive material, projecting a light image upon said charged surface thereby producing an electrostatic image corresponding to said light image, developing said electrostatic image with a finely-divided developer substance in configuration with said electrostatic image, fixing said finely divided developer substance substantially in situ, dissolving substantially all of said dispersed photoconductor from said photoconductive material with a reagent which is a solvent for said photoconductor but not for said organic film-forming binder, said organic film-forming binder and said fixed developer substance being substantially unaffected by said reagent, rinsing the remaining material, and then drying said rinsed material.

5. An electrostatic printing process comprising producing a blanket electrostatic charge upon the surface of a photoconductive material which comprises a photoconductor dispersed in an organic, film-forming binder, projecting a light image upon said charged surface thereby producing an electrostatic image substantially corresponding to said light image, developing said electrostatic image with a finely-divided developer substance, fixing said finely divided developer substance substantially in situ, dissolving substantially all of said dispersed photoconductor from said photoconductive material with a reagent which is a solvent for said photoconductor but not for said organic film-forming binder, said organic film-forming binder and said fixed developer substance being substantially unaffected by said reagent, rinsing the remaining material and then drying said rinsed material.

6. An electrostatic printing process comprising producing an electrostatic image on a photoconductive material which comprises a zinc oxide dispersed in an organic,

nlm-forming binder, developing said electrostatic image with a finely-divided developer substance, fixing said linely divided developer substance substantially in situ and then dissolving substantially all of said oxide from said photoconductive material with a reagent which is a solvent for said zinc oxide `but not for said organic filmforming binder, said organic tilmaforming binder and said developer substance -being substantially unalected by said reagent.

7. An electrostatic printing process comprising producing an electrostatic image on a photoconductive material which comprises zinc oxide dispersed in a silicone resin, developing said electrostatic image with a iinely-divided developer substance, fixing said finely divided developer substance substantially in situ and then dissolving substantially all of said zin'c oxide from said photoconductive material with a reagent which is a solvent for said zinc oxide but not for said silicone resin, said silicone resin and said lixed developer substance being substantially unaiTected by said reagent.

8. An electrostatic printing process comprising producing an electrostatic image on a photoconductive material which comprises a zinc sulphide dispersed in an organic, film-forming binder, developing said electrostatic image with a finely-divided developer substance, fixing said nely divided developer substance Substantially in situ and then dissolving substantially all of said zinc sulphide from said photoconductive material with a reagent which is a solvent for said zinc sulphide but not for said organic nlm-forming binder, said organic film-forming binder and said iixed developer substance being substantially unaf fected by said reagent.

9. An electrostatic printing process comprising producing an electrostatic image on a photoconductive material which comprises zinc sulphide dispersed in a silicone resin, developing said electrostatic image with a finely divided developer substance, fixing said finely divided developer substance substantially in situ and then dissolving substantially all of said zinc sulphide from said photoconductive material with a reagent which is a solvent for said zinc sulphide but not for said silicone resin, said silicone resin and said fixed developer substance being substantially unaffected by said reagent.

10. An electrostatic printing process comprising producing an electrostatic image on a photoconductive material which comprises a cadmium sulphide dispersed in an organic, film-forming binder, developing said electrostatic image with a finely-divided developer substance, fixing said finely divided developer substance substantially in situ and then dissolving substantially all of said cadmium sulphide from said photoconductive material with a reagent which is a solvent for said cadmium sulphide but not for said organic film-forming binder, said organic film-forming binder and said fixed developer substance being substantially unalected by said reagent.

11. An electrostatic printing process comprising producing an electrostatic image on a photoconductive material which comprises cadmium sulphide dispersed in a silicone resin, developing said electrostatic image with a finely-divided developer substance, fixing said iinely divided developer substance substantially in situ and then dissolving substantially all of said cadmium sulphide from said photoconductive material with a reagent which is a solvent for said cadmium sulphide but not for said silicone resin, said silicone resin and said iixed developer substance being substantially unaiected by said reagent.

References Cited in the file of this patent UNITED STATES PATENTS 1,939,213 Jelley Dec. 12, 1933 2,297,691 Carlson Oct. 6, 1942 2,386,626 Nadeau et al Oct. 9, 1945 2,425,363 Crabtree et al. Aug. l2, 1947 2,663,636 Middleton Dec. 22, 1953 FOREIGN PATENTS 402,737 Great Britain Mar. 5, 1932 OTHER REFERENCES Wainer: Phot. Eng., vol. 3, No. 1, 1952, pages 12 to 18. 

1. IN AN ELECTROSTATIC PRINTING PROCESS WHICH COMPRISES PRODUCING AN ELECTROSTATIC IMAGE ON A PHOTOCONDUCTIVE MATERIAL COMPRISING A PHOTOCONDUCTOR DISPERSED IN AN ORGANIC FILM-FORMING BINDER, DEVELOPIND SAID ELECTROSTATIC IMAGE WITH A FINELY-DIVIDED DEVELOPER SUBSTANCE AND FIXING SAID FINELY DIVIDED DEVELOPER SUBSTANCE SUBSTANTIALLY IN SITU, THE STEP SUBSEQUENT TO SAID FIXING COMPRISING SUBJECTING SAID PHOTOCONDUCTIVE MATERIAL TO A REAGENT WHICH IS A SOLVENT FOR SAID PHOTOCONDUCTOR AND NOT FOR SAID BINDER UNTIL SUBSTANTIALLY ALL OF SAID PHOTOCONDUCTOR IS DISSOLVED, SAID ORGANIC, FILM-FORMING BINDER AND SAID FIXED DEVELOPER SUBSTANCE BEING SUBSTANTIALLY UNAFFECTED BY SAID REAGENT. 